Superconductors

ABSTRACT

A METHOD OF MAKING A SUPERCONDUCTOR COMPRISES COLD REDUCTION OF AT LEAST 10%, HEAT-TREATING 100*C.-500*C., AT LEAST ONE FURTHER COLD REDUCTION OF AT LEAST 5%, AND HEAT-TREATING 100*C-500*C.

United States Patent O 3,715,243 SUPERCONDUCTORS Anthony Robert King Clayton, Sutton Coldfield, and Anthony Clifford Barber, Lichfield, England, assignors to Imperial Metal Industries (Kynoch) Limited, Birmingham, England No Drawing. Filed Apr. 27, 1967, Ser. No. 634,072 Claims priority, application Great Britain, May 4, 1966, 19,756/ 66 Int. Cl. C22f 1/18 US. Cl. 148-115 R 11 Claims ABSTRACT OF THE DISCLOSURE A method of making a superconductor comprises cold reduction of at least 10%, heat-treating 100 C.500 C., at least one further cold reduction of at least 5%, and heat-treating 100 C.500 C.

BACKGROUND OF THE INVENTION This invention relates tosuperconductors comprising a ductile superconductor material, particularly an alloy of niobium and titanium or an alloy of niobium, titanium and zirconium, and more particularly to a method of heattreating such superconductors.

Alloys of niobium, containing titanium or titanium and zirconium have been proposed as superconductors either unsheathed, or sheathed in single or multiple filamentary form in a ductile non-superconductor stabilising material, e.g. high conductivity copper. These alloys may be cold worked, such as by drawing or rolling, to a cold reduction of at least 90% followed by heat-treatment at 100 C.- 500 C., thereby improving the performance of the superconductor in respect of critical current density.

We have found that good superconducting properties can be obtained by a modification of the process of manufacture outlined above.

SUMMARY OF THE INVENTION Accordingly the present invention consists in a method of manufacturing a superconductor comprising a first step of cold working a ductile superconductor material to effect a reduction in the cross-sectional area of the material of at least and then heat-treating the material at about 100 C.-500 C. for at least ten minutes to produce a primary product, and at least one further step of cold working the primary product to effect a reduction in cross-sectional area of at least 5% and then heattreating at 100 C.-500 C. for at least ten minutes for each of the further steps, there being a minimum total reduction in the cross-sectional area of the material of 95%.

The minimum total reduction in area is at least 95 but the effect of the combined treatments on the superconducting properties differs from the effect of a single cold working treatment of at least 95% followed by heattreatment.

A single inter-working heat-treatment shows a substantial change in properties, but a marked improvement takes place when two or more steps are carried out. For an explanation of these unexpected properties, some consideration of the characteristics of the superconducting materials involved is necessary.

The ductile superconducting materials referred to above are not capable of carrying any useful electrical current When in a completely homogeneous, strain-free state, so that in order to achieve useful supercurrent densities, it is thought necessary to introduce into the material inhomogeneities of composition and/or of strain which are then though to act as pinning centres to stabilise the magnetic flux. Thus heavy cold working is necessary to bring about 3,715,243 Patented Feb. 6, 1973 satisfactory current densities, as this introduces into the atomic lattice a large number of dislocations which are thought to act as pinning centres. An aging treatment following cold working apparently renders the network of dislocations more efficient in this respect.

We have found that a second cold working step fol lowed by aging appears to modify the nework and render it even more efiicient in the distribution of pinning centres. As a result, therefore, of the improved distribution of pinning centres, the supercurrent density of the material is likewise increased.

Preferred compositions of ductile superconducting alloys to which the method of the invention may be applied are 10-60 wt. percent titanium, 0.40 wt. percent zirconium, 0-1500 p.p.m. oxygen, 0-2000 p.p.m. nitrogen and 04000 p.p.m. carbon, balance niobium.

Large reductions in area of at least 99.9% are preferred and in order, inter alia, to facilitate such reductions the superconducting material may be contained in high conductivity copper, extruded and drawn to wire, or simply drawn to wire.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One example of the method in accordance with the invention will now be more particularly described.

An ingot of Nb-44Ti was forged at a temperature of at least 700 C., quenched to room temperature, and then machined to give a cylindrical slug which was placed in a copper container such that in section the ratio of areas of copper and niobium-titanium was 3: 1. This was done as it is known that the presence of the copper improves the properties of coils made from such wire owing to its acting as a heat sink, annulling the heating effect of flux jumps, and also as an outer layer of copper markedly improves the fabricability of the superconducting wire. The container with niobium-titanium insert was then evacuated and sealed (not an essential step) before being heated at 450 C. for V2 hour, and extruded at a 7:1 area ratio whilst still warm to produce a good bond between the copper and superconductor. This rod was then cold drawn to a 99.82% reduction in area.

Two lengths were then taken; the first was cold drawn a further 90.23%, giving a total reduction in area of 99.98%, and then heat-treated for 1 hour at 400 C. to produce specimen A; and the second was heat-treated for /2 hour at 350 C. to produce a primary product, and given the same heat-treatment between each pass (18 in all) during cold drawing a further 90.23%, and finally given a heat-treatment for 1 hour at 350 C. to produce specimen B.

The results of tests performed on these specimens is given in the table below:

Applied field 20 25 30 35 40 45 (kGauss) kg. kg. kg. kg. kg. kg.

Criticalcurrentdensity {A 24.8 21.5 19.3 17.1 15.6 14.1 (X10 a./cm.) B 30.0 26.0 2.92 20.5 18.4 16.1

A: cold drawn 99.98%; 1 hour 400 C. B: cold drawn 99.82%; /2 hour 350 C. between each of 18 passes on cold drawing 90.23%; 1 hour 350 C.

Specimen C was cut from the drawn rod, drawn at ambient temperatures by a further 99.87% to give an overall reduction of 99.97%, heat-treated for 1 hour at 375 C., and its critical current density tested subsequently. Specimen D was also cut from the drawn rod, and after 1 hour at 500 C., to produce a primary product, was given the same reduction of 99.87% and final heat-treatment of 1 hour at 375 C. as specimen C. It was also tested subsequently.

Specimen E was taken from specimen C and was cold reduced by 11%. Specimen F was taken from specimen E and was heat-treated for 1 hour at 375 C. Specimens E and F were then tested, and the results are tabulated below with those of specimens C and D.

Thus the results for specimen D show the effects, which are substantial, of a single interstage anneal, and despite the reduction in properties produced by further working, as reflected in the results for specimen- E, those for specimen F show the large benefits accruing from two interstage anneals.

We claim:

1. A method of manufacturing a superconductor comprising a first step of cold working a ductile superconductor alloy containing -60 weight percent titanium, up to 40 weight percent zirconium, balance niobium to effect a reduction in the cross-sectional area of at least 10% and then heat-treating the alloy at about 100 C.- 500 C. for at least ten minutes to produce a primary product, and at least one further step of cold working the primary product to effect a reduction in cross-sec tional area of at least 5% and then heat-treating at 100 C.500 C. for at least ten minutes for each of the further steps, there being a minimum total reduction in the cross-sectional area of the material of 95%.

2. A method according to claim 1 wherein there are at least two of said further steps.

3. A method according to claim 1 wherein the minimum total reduction in the cross-sectional area of the material is 99.9%.

4. A method according to claim 1 wherein said first step effects a reduction of about 99.82% and the alloy is then heat-treated for about /2 hour at about 350 C. to produce a primary product, and the primary product is heat-treated for about /2 hour at about 350 C. between each one of about eighteen passes during cold drawing a further 90.23% approximately, followed by a heat-treatment for about 1 hour at about 350 C.

5. A method according to claim 1 wherein said first 4 step effects a reduction of about 89.7% and the alloy is then heat-treated for about 1 hour at 500 C. to produce a primary product, and the primary product is reduced by 99.87% by cold working and then treated at about 375 C. for about 1 hour.

6. A method according to claim 5 further comprising an additional reduction of about 11% by cold working and then a heat-treatment for about 1 hour at about 375 C.

7. A superconductor manufactured in accordance with the method of claim 1.

8. A method as in claim 1 wherein the alloy contains up to 1500 p.p.m. oxygen, up to 2000 p.p.m. nitrogen and up to 1000 p.p.m. carbon.

9. The method of producing a superconductor comprising: solidifying from the molten state a niobium alloy containing 10-60 weight percent titanium and up to 40 weight percent zirconium; cold working the solidified alloy to effect a reduction in cross-section of at least 10%; heat treating the alloy at about 100 C.500 C. for at least ten minutes to produce a primary product; and improving the superconducting properties by alternately cold working and heat treating said primary product a plurality of times to effect at least 5% reduction in cross section upon each cold working step and to effect a minimum total reduction in the cross-section of each of said heat teratments being carried out at C.500 C. for at least ten minutes.

10. A method of increasing the critical current density of a niobium-titanium superconductor which comprises cold reducing the cross-sectional area of a niobiumtitanium body by at least 10%, heat-treating said cold worked body between about 100 C. and 500 C. for a time on the order of ten minutes to one hour, further cold working the heat treated body to effect a reduction of cross-sectional area of at least 5%, heat treating the further reduced product at 100 to 500 C. for a time on the order of ten minutes to one hour and further cold reducing the heat treated product such that the minimum total reduction in the cross-sectional area of the material is 95%.

11. The method of claim 10 wherein the finally cold reduced superconductor is given a heat treatment between about 100" C. and 500 C. for a time of between ten minutes and one hour.

References Cited UNITED STATES PATENTS 3,268,373 8/1966 Reynolds l48ll.5 3,271,200 9/1966 Zwicker 14811.5 3,476,615 ll/l969 Fairbanks et al. 148-l2.7 3,275,480 9/1966 Betterton, Jr. et al. 148-11.5

WAYLAND W. STALLARD, Primary Examiner US. Cl. X.R. 148-l2.7 

